1. Field of the Invention
The present invention relates to an actuator comprising a reversible motor, a transmission operatively connected to the motor, a movable adjustment element operatively connected to the transmission, a cylindrical part, a coil spring arranged on the cylindrical part and with the direction of winding such that the spring exerts a braking effect on the adjustment element in the one direction of movement thereof in that the spring is tightened around the cylindrical part, said braking effect being adapted such that it may be overcome by the motor.
2. The Prior Art
EP 0 662 573 to Linak A/S discloses a linear actuator, where a reversible electric motor, via a transmission, drives a spindle with a nut secured against rotation. A tubular adjustment element is secured to the nut, whereby the adjustment element is displaced axially between an extended position and a retracted position when the spindle is rotated one way and the other way, respectively. The actuator is to have a sufficient self-locking ability for the adjustment element to remain in the position concerned when the motor is stopped. To achieve the sufficient self-locking ability, EP 0 662 573 teaches to arrange a coil spring, whose one end is secured, around a rotating element in the transmission to exert a braking force, so that the actuator is self-locking. The screw is configured and arranged so as to exert a braking force in the inward direction of the adjustment element, as the friction between the rotating element and the spring causes the spring to contract around the rotating element, and to open, i.e. loosen its grip around the rotating element, in the outward direction of the adjustment element. Since its appearance in 1995, the structure has been widely used not only in single actuators, as shown in EP 0 662 573, but also in dual actuators, as shown in DE 38 42 078 A1 to Niko Gesellschaft für Antriebstechnik mbH, and EP 1 014 834 to Okin Gesellschaft für Antriebstechnik mbH & Co. KG, as well as in actuators constructed as lifting columns, including lifting columns for height-adjustable tables, cf. e.g. WO 02/39848 A1 to Linak A/S. The structure has also been described later on by a competitor in DE 296 06 367 U1 to Okin Gesellschaft für Antriebstechnik mbH.